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The unveiling on Wednesday morning of the Senate's long-awaited draft legislation to reduce U.S. carbon emissions and shift the country to a clean-energy economy signals that Washington is inching ever closer to addressing global warming. The sweeping bill, sponsored by Democratic Senators Barbara Boxer and John Kerry, will cut U.S. greenhouse-gas emissions 20% below 1990 levels by 2020, and 83% by 2050  targets that in the short term are a bit more ambitious than a similar carbon cap-and-trade bill passed by the House two months ago. "This is the beginning of one of the most important battles we will face, as legislators, as citizens," Kerry said Wednesday, flanked by veterans, local legislators and clean-energy entrepreneurs. "It is time to reinvent the way America uses energy."

President Barack Obama issued a similar call to action when he spoke to the U.N. about climate change at a daylong summit on Sept. 22. But his speech, which contained high emotion and few concrete specifics on how much the U.S. would actually cut greenhouse-gas emissions, disappointed some. It was remarkable, after eight years of stonewalling from former President George W. Bush, to see a U.S. leader rally the rest of the world to combat global warming, but Obama kept his carbon promises vague, suggesting his limits. (Read "A Wind Shift in the Global Warming Debate?")

Congress is one of them. The Kerry-Boxer bill is only one of several pieces of legislation that the Senate will need to consider as it takes on cap and trade  about which the Finance Committee is only one powerful group that will have its say  and the chances that any kind of carbon cap will pass seem vanishingly small. As long as the Senate is stuck on other business, like health care, Obama and his negotiators will have their hands essentially tied at the U.N. climate-change summit in Copenhagen three months from now. They can't commit the U.S. to carbon cuts internationally if the Senate won't support them at home. That was the pitfall former President Bill Clinton failed to avoid with regard to the Kyoto Protocol  and Obama won't repeat his mistake.

In the absence of congressional action, the President does have other options. The Environmental Protection Agency (EPA) on Wednesday announced a proposed rule that would regulate carbon emissions from large emitters  primarily power plants  that emit 25,000 tons of greenhouse gases or more. The rule is the latest step in the EPA's response to a 2007 Supreme Court case that classified carbon dioxide and other greenhouse gases as pollutants that required EPA regulation under the Clean Air Act. The proposed rule will oblige those large emitters to get permits that demonstrate they are using the best available technology for controlling carbon whenever they engage in new construction or upgrading. "By using the power and authority of the Clean Air Act, we can begin reducing emissions from the nation's largest greenhouse-gas emitters without placing an undue burden on the businesses that make up the vast majority of our economy," said EPA administrator Lisa Jackson in a speech at the Governors' Global Climate Change Summit in Los Angeles.

The EPA's proposed rule marks the first time the Federal Government has tried to regulate stationary sources of greenhouse-gas emissions. But again, the details are far from certain. It's not clear yet what "best available technology" will mean for carbon  especially in the case of new coal plants, which have no real way to drastically limit carbon emissions. And the rule is certain to come under attack from industry opponents; by putting only large emitters under the proposed rule, the EPA saves a lot of expense for small businesses but could be accused of being unfair to larger ones.

As for the Kerry-Boxer bill, most environmental groups supported it, happy to see a bill with a tighter short-term emissions-reduction target. But the bill still has plenty of holes for a piece of legislation that has been in the works for months, saying little about how allowances for carbon emissions would be distributed among polluting industries  a key part of any cap-and-trade bill. And with natural gas fans blaming the bill for having too little support of natural gas and nuclear fans saying there's not enough support of nuclear, gathering up support from Republican Senators and even Democrats will be an uphill challenge. The negotiations are just beginning.

For their part, more-radical environmental groups, including Greenpeace, withheld support from the Kerry-Boxer bill  as they did the House cap-and-trade bill  saying its carbon cuts were far too modest to save the climate. Scientifically, they're probably right  the Intergovernmental Panel on Climate Change has said that developed nations like the U.S. need to cut carbon emissions 25% to 40% by 2020 to keep global warming within what is hoped are safe limits. Politically, however, that seems out of question for Congress. Which is why Obama's speech at the U.N. last week was an exercise in realism, even if much of his audience wanted idealism.

It might be a battle between politics and science going forward as carbon keeps getting pumped into the atmosphere and the days remaining before Copenhagen tick away. But as U.N. Secretary-General Ban Ki-moon told a group of reporters on Tuesday, "You can't negotiate with nature. [Climate change] is just coming." Not even the Senate can argue with that.